Preliminary results with a free piston shock tunnel

Abstract

O widely used means of studying high-velocity gas flows is the shock tunnel, which involves a hypersonic nozzle supplied with test gas heated in a shock tube. The speed of the shock in the tube governs the nozzle stagnation enthalpy, and because present techniques limit this speed to about 11,000 fps, maximum enthalpies obtainable correspond to nozzle velocities of about 16,000 fps. The free piston shock tunnel represents an attempt to increase these enthalpies by raising the shock speed. It derives from the free piston shock tube, in which the shock is produced by a driver gas, initially contained in a large tube at relatively low pressure, which is isentropically heated and raised to high pressure by a single stroke of a relatively heavy free piston. The piston speed is sufficiently low that conditions are essentially uniform throughout the gas at any instant during compression. The shock tunnel modification is illustrated in Fig. la and is explained in detail in Ref. 2; only a brief description is given here. The shock tube diaphragm ruptures before the piston compression stroke is complete, at a moment when the piston velocity is such that subsequent movement of the piston face approximately compensates for the flow of gas from the driver volume into the shock tube. Provided that the area ratio of the driver compression tube to the shock tube is large enough to neglect the expansion wave generated in the driver by the bursting diaphragm, the driver conditions then remain approximately constant for a period, which is prolonged by the piston movement. This period is ultimately limited by the need to allow the piston to finally slow down and come to rest before striking the end of the tube. However, with a driver gas of 7 = 1.67, it is theoretically posa)